It is known that high intensity acoustic pulses may be produced by deliberately creating a "water hammer". A water hammer is the high pressure pulse created when a rapidly flowing stream of fluid in a conduit is suddenly blocked. When this occurs, for example by the sudden closing of a valve, the kinetic energy of the flowing fluid is converted to a high pressure pulse. The high pressure pulse propagates upstream from the valve at a velocity which is a function of the speed of sound in the fluid in the pipe and the dimensions and elasticity of the pipe. A similar phenomenon results in a reduced pressure pulse being propagated downstream away from the valve.
When the pressure pulse arrives at a point in the conduit upstream of the valve, the fluid at that point stops flowing toward the valve. The increased pressure forces the walls of the conduit outwardly and compresses the fluid within the conduit. The motion of the walls of the conduit can create acoustic waves in a fluid medium outside of the conduit.
Fluid at a point upstream from the valve is not affected by the closing of the valve until the pressure pulse reaches that point. The mathematics of water hammer are discussed in various texts on fluid mechanics including Fluid Mechanics (7th Edition) Victor L. Streeter and E. Benjamin Wylie, McGraw-Hill Book Company, 1979.
It is known that the water hammer effect can be used to generate acoustic pulses for use in marine seismic exploration. Baker et al., U.S. Pat. No. 3,376,949, discloses an acoustic generator which utilizes the water hammer effect to create acoustic pulses in a fluid. The acoustic generator comprises a high energy pump for providing a stream of rapidly flowing fluid, a downwardly extending pipe through which the fluid is pumped, and a valve at the lower end of the pipe. A zone in the pipe upstream from the valve is perforated and surrounded by a heavy rubber tube.
The Baker et al. device is used by submersing the lower end of the downwardly extending pipe in a body of water and pumping fluid through the pipe at high velocity. When an acoustic pulse is desired, the valve is suddenly closed, thereby arresting the flow of fluid in the pipe and causing a water hammer within the pipe.
The water hammer pressure pulse created when the rapidly flowing fluid is brought to a halt by the closed valve propagates up the pipe toward the pump. Because the rubber tube is compliant, the pressure pulse forces the rubber tube outward as it travels up the pipe. The result is that the rubber tube displaces the surrounding water, thereby generating acoustic waves in the water surrounding the tube. A single short burst of acoustic waves is produced each time the valve is closed.
Anstey, U.S. Pat. No. 3,536,157 discloses an underwater acoustic generator, also for use in underwater seismic exploration, which is designed to be towed behind a boat. The Anstey generator comprises a length of conduit with a velocity transformer at each end and a normally-open valve near the trailing end of the conduit. When the Anstey device is towed through the water with the valve open, the velocity transformers direct a high velocity stream of water through the conduit. When the valve is suddenly closed a water hammer is set up inside the conduit. The water hammer pressure pulse travels along the conduit away from the valve. The walls of the conduit in on embodiment of the Anstey device are partially compliant so that the pressure pulse causes acoustic energy to radiate from the conduit as it travels along the conduit.
Burg, U.S. Pat. No. 4,271,925 discloses another system for generating acoustic pulses for use in seismic exploration by deliberately creating a water hammer effect within a conduit. The Burg system comprises a pressure vessel for providing a source of pressurized fluid, an acoustically transparent conduit connected to the outlet of the pressure vessel, an upstream valve between the pressure vessel and the conduit for controlling the flow of fluid from the pressure vessel into the conduit and a downstream valve at the outlet of the conduit for abruptly terminating fluid flow in the conduit.
The Burg device is used to generate an acoustic pulse by the steps of pressurizing the fluid within the pressure vessel, opening the upstream valve to allow high pressure fluid to flow into the conduit, waiting until the fluid in the conduit is flowing at a maximum velocity, and then suddenly closing the downstream valve. When the downstream valve is closed, a water hammer pressure pulse is created within the conduit. The high amplitude pressure pulse is reflected back and forth through the conduit between the upstream pressure vessel and the downstream valve. The result is that an acoustic signal pulse with a characteristic frequency dependent upon the length of the conduit is radiated away from the conduit each time the valve is closed.
Other references which disclose means for harnessing water hammer to generate acoustic waves in a fluid are Bricout, U.S. Pat. No. 3,369,519 and Davis, U.S. Pat. No. 3,690,403.
Each of the references above discloses the use of a deliberately created water hammer to produce a one-shot high amplitude burst of acoustic signals appropriate for geophysical seismic exploration. In each case, the acoustic signals are radiated from a circularly symmetrical conduit. None of the references above disclose an acoustic source having means for rapidly cycling a valve open and closed to yield a series of water hammer pulses in a conduit to produce a continuous acoustic signal with a characteristic frequency dependant upon the rate of valve operation.
Bayhi, U.S. Pat. No. 4,396,088 discloses a generator of low power, low frequency acoustic waves. The Bayhi apparatus modulates the flow of fluid flowing into an array of flexible sleeves at the desired frequency. Bayhi does not disclose the use of water hammer to generate high amplitude acoustic waves.
Acoustic cleaners using various piezoelectric, magneto-strictive or voice coil transducers are known in the prior art. For example, Japanese patent No. 63318993 discloses a clothes washing machine which uses sonic energy to clean clothes. The sonic energy is generated by means of a voice coil which vibrates one wall of a compartment containing the clothes.
An example of the use of ultrasonic waves for cleaning items is the ultrasonic cleaning bath which is commonly used in laboratories for cleaning small items. Such prior art ultrasonic cleaning baths operate at high frequencies, typically above 20,000 Hz, and at acoustic intensities which are typically much lower than the acoustic power levels developed by the present invention. The acoustic signals in ultrasonic cleaning baths are typically developed by means of piezoelectric or magneto-strictive transducers. These prior art ultrasonic transducers are essentially pistons which can be reciprocated very rapidly to generate an acoustic signal. When such prior art ultrasonic transducers are operated at high intensities cavitation can occur in the fluid near the ultrasonic transducer during the retraction phase of the piston. The cavitation bubbles interfere with the propagation of the ultrasonic beam. It is therefore difficult to generate high intensity acoustic signals using such transducers.
A further disadvantage of prior art ultrasonic transducers in applications where an object or body of fluid is to be treated at a distance from the transducer is that the attenuation of high frequency acoustic waves in typical fluids is much greater than the attenuation of lower frequency acoustic waves in the same fluids. Therefore, high intensity compressional pulses delivered at comparatively low frequency can be more effective for treating materials at long distances from a transducer than high frequency, relatively low intensity ultrasonic waves.
The disadvantages of prior art transducers is compounded in some applications by dispersion effects. As an intense, high frequency acoustic wave travels in a fluid, adjacent pressure peaks in the wave tend to spread out and merge with each other. This effect can cause particular problems at ultrasonic frequencies because at such frequencies the wavelength is short and adjacent pressure peaks are close together. The effectiveness of an acoustic wave for cleaning or for other applications can be reduced by spreading of the acoustic pressure peaks in the wave. These dispersion effects could be reduced by increasing the distance between the pressure peaks in the acoustic wave (i.e. by lowering the frequency of the acoustic wave). However, lowering the frequency of a sinusoidal acoustic wave, which is the type of acoustic wave most commonly produced by prior art transducers, while holding the intensity of the wave constant has the effect of increasing the rise time for each of the pressure pulses in the acoustic wave. This, in turn, may reduce the effectiveness of the acoustic wave for cleaning or for other applications. Ideally, a generator of acoustic waves for cleaning or other material treatment applications would be capable of producing distinct pressure pulses, with very fast rise times, separated by a selected interval.
Water hammer has been used for cleaning the inside of pressure vessels and tubes. For example, Canadian patent No. 837971 discloses a method for cleaning a heat exchanger tube. The method comprises the steps of passing fluid through the heat exchanger tube, heating the fluid to near its boiling point and suddenly and repeatedly interrupting the flow of the fluid at the tube inlet. Interrupting the fluid flow causes low pressure pulses to propagate through the tube. The low pressure pulses clean the inner surface of the tube by causing the fluid to boil at the tube's inner surface. Karpovich, U.S. Pat. No. 3,409,470 discloses a similar method. These references do not disclose the use of water hammer for cleaning objects in a medium external to the tube in which the water hammer is generated.
Acoustic fields have also been used for removing sediment from fluids. For example, United Kingdom patent No. 2098498 discloses a system for removing sediment from a flowing fluid which comprises a pair of opposed ultrasonic transducers for generating a drifting standing wave across the direction of fluid flow. The standing wave sets up pressure gradients in the fluid which collect particles of sediment. As the standing wave drifts, the collected particles of sediment are swept into a collection zone.
A problem with the prior art water hammer acoustic generators discussed above is that they are designed to produce single bursts of acoustic waves for seismic exploration. A one-shot acoustic generator is not optimal for acoustic cleaning, acoustic sedimentation or similar uses where a substantially continuous acoustic signal is desirable to minimize processing times.
A further disadvantage of the prior art water hammer acoustic generators described above is that these generators emit acoustic waves in a pattern which is symmetrical about the axis of the conduit in which water hammer is created. An axially symmetric radiation field is useful in seismic exploration but is wasteful of acoustic energy in situations where it is desired to concentrate acoustic waves to treat a volume of fluid or a workpiece.